1. Field of the Invention
The present invention relates to a driving method of a semiconductor device using a semiconductor element.
2. Description of the Related Art
Storage devices using semiconductor elements are roughly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even without being supplied with power.
A typical example of a volatile storage device is a DRAM (dynamic random access memory). A DRAM stores data in such a manner that a transistor included in a memory element is selected and electric charge is stored in a capacitor.
According to the above-described principle, once data is read from a DRAM, electric charge in the capacitor is lost; thus, a writing operation needs to be performed every time data is read out. Moreover, the transistor in the memory element has a leakage current (off-state current) between a source and a drain in an off state, and therefore, electric charge flows into or out of a capacitor even when the transistor is not selected, whereby a data retaining period is short. For that reason, a writing operation (refresh operation) needs to be performed at certain intervals, which makes it difficult to sufficiently reduce power consumption. Further, since stored data is lost when power supply stops, another kind of storage device using a magnetic material or an optical material is needed in order to retain stored data for a long time.
Another example of a volatile storage device is an SRAM (static random access memory). An SRAM retains stored data by using a circuit such as a flip-flop and thus does not need a refresh operation, which is an advantage of the SRAM over the DRAM. However, cost per storage capacity is increased because the circuit such as a flip-flop is used. In addition, as in the DRAM, stored data in the SRAM is lost when power supply stops.
A typical example of a non-volatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by retaining electric charge in the floating gate. Therefore, a flash memory has advantages in that the data retaining period is extremely long (almost permanent) and a refresh operation, which is necessary in a volatile storage device, is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a memory element deteriorates by tunneling current generated in writing data, a number of writing operations deteriorate the function of the memory element. In order to reduce adverse effects of this problem, a method in which the number of writing operations for each memory element is equalized is employed, for example; however, a complicated peripheral circuit is involved to carry out this method. Further, such a method does not solve the fundamental problem of lifetime. That is, a flash memory is not suitable for applications in which data is frequently rewritten.
As for the flash memory, a “multi-valued” flash memory in which data whose number of levels is greater than 2 is stored in one memory cell has been proposed for higher memory capacity (e.g., see Patent Document 2).